A polymer electrolyte fuel cell has a structure in which catalyst layers are arranged on respective surfaces of a polymer electrolyte membrane, and a gas diffusion layer is arranged on the outer side of each catalyst layer. In general, the catalyst layer is a porous layer made of a catalyst-supporting carrier in which a noble metal catalyst is supported on the surface of carrier particles. A fuel gas such as hydrogen or methanol, or an oxidant such as oxygen or air, circulates through the porous layer, and an electrode reaction occurs at the three-phase interface, producing water within the catalyst layer.
The produced water dissipates from the catalyst layer, but in some cases, the water accumulates within the catalyst layer. If this progresses, the catalyst layer will not be able to hold the water therein, giving rise to a phenomenon called flooding. With the aim of preventing flooding, Patent Literature 1 proposes to provide a cathode catalyst layer of a fuel cell with sections for reducing oxygen and sections having a higher water repellency than the oxygen-reducing sections. When the surface of the cathode catalyst layer is observed, the highly water-repellent sections are distributed unevenly.
The fuel cell disclosed in Patent Literature 1 employs a carbon material as a catalyst carrier. In cases where, for example, a fuel cell is used so as to be activated and stopped repeatedly in a short time, carbon materials are known to oxidize and corrode during voltage fluctuation or during the stoppage of supplied gas. With the aim of overcoming this drawback of carbon materials, proposals have been made to use materials other than carbon materials, such as metal oxides, as the catalyst carrier.
The surface of a metal oxide, however, has high wettability to water, which makes flooding more likely compared to carbon materials. Patent Literature 2 provides a carrier made of an inorganic substance with water repellency by causing a water-repellent surface protection substance to be adsorbed on the carrier surface. Examples of the water-repellent surface protection substance include long-chain organic acids such as stearic acid, silica-based materials, and fluorine-based materials.